Fix daemon mode in multiple programs under OSX

Affected programs:
  - seaf-controller
  - seaf-daemon
  - fileserver
  - seaf-server

the reason why we couldn't use ``daemon`` under OSX is that ``daemon`` is deprecated.
 
the line below is from ``/usr/include/stdlib.h`` from [libc shipped along with OSX 10.9.2](http://opensource.apple.com/tarballs/Libc/Libc-997.90.3.tar.gz)
```
int	 daemon(int, int) __DARWIN_1050(daemon) __OSX_AVAILABLE_BUT_DEPRECATED(__MAC_10_0, __MAC_10_5, __IPHONE_2_0, __IPHONE_2_0);
```